The technology of "nondestructive testing" allows structural examination of devices and materials without destruction or disassembly of the device or material under test. Nondestructive testing is commonly employed to detect unsafe or potentially unsafe conditions, such as cracks, voids, holes and structural flaws in metals, plastics, and composite materials and devices made therefrom. Nondestructive testing has found application to both on-line inspection at point of material manufacture and on-site testing of installed products. Instrument mobility is a particularly important consideration to on-site nondestructive testing.
One method of nondestructive testing utilizes ultrasonic instrumentation which electrically stimulates a contact transducer. The electrical stimulus excites the contact transducer which responds by oscillating at an ultrasonic frequency. When the contact transducer is acoustically coupled to a material or device to be tested, the contact transducer excites that material or device as well, so that ultrasonic vibrations travel through the material or device. Reflections, or echoes, of the incident vibrations from defects are processed by instrumentation which indicates locations and/or sizes of the defects on a visual display, such as a cathode ray tube.
It is known in the prior art to employ a piezo ceramic material for the contact transducer. Examples of such prior art contact transducers are the "Accuscan" and "Videoscan" transducers manufactured by Panametrics of Waltham, Massachusetts. A problem with piezo ceramic contact transducers, however, is that they are typically thick, bulky and inflexible, and do not acoustically match well with most composite materials. Since these transducers are inflexible, they are not suitable for use on surfaces that are curved or complex in shape. Additionally, since they are bulky, these transducers are not well suited for mobile use.
It is also known in the prior art to employ a piezo film material for the contact transducer. One example of a prior art nondestructive testing apparatus which utilizes piezo film contact transducers is the Portable Automated Remote Inspection System (PARIS) manufactured by Failure Analysis Associations, Inc. of Redmond, Wash., a subsidiary of Sigma Technologies Corporation. PARIS employs large area flexible transducer arrays which comprise, for example, 1024 addressable transducer elements that are configured in a 32.times.32 array in a "blanket" configuration. While piezo film contact transducers are generally more adaptable than their piezo ceramic counterparts, the blanket of the PARIS system is bulky, heavy and must be vacuum sealed. Further, it is not readily deformable, it must be addressed by a computer, it cannot be permanently adhered to the surface of the material under test, it will not fit into tight places, and it is not disposable or expendable.
Piezo film contact transducers are also manufactured by the assignee of the present invention, Pennwalt Corporation, under the trademark Kynar.RTM.. The DT, LDT, BDT, SDT and FDT family of KYNAR.RTM. transducers are exemplary. Model number LDTl-028K is typical of Pennwalt's Kynar.RTM. piezo film contact transducers. It is constructed from a 28 .mu.m-thick layer of poled polyvinylidene fluoride (PVDF) that is laminated to a 5-mil layer of Mylar.RTM. (a registered trademark of DuPont), and protected by a screenprinted clear polymer coating made of fluoropolymers, urethanes or acrylics, or by an acrylic-adhesive backed polyester tape such as 3M #850 tape. More detailed information relating to particular piezo film contact transducers of this type is found in the "Kynar.RTM. Piezo Film Product Summary and Price List" (1988) available from Pennwalt Corporation of Philadelphia, Pa. Additional information relating to the structure, properties, application and fabrication of Kynar.RTM. piezo film contact transducers is found in the "Kynar.RTM. Piezo Film Technical Manual" (1987), also available from Pennwalt Corporation. Both of these publications are incorporated herein by reference.
Notwithstanding the great extent to which Kynar.RTM. piezo film contact transducers have been successfully used, these transducers suffer from several disadvantages in their application as ultrasonic contact transducers. For example, they are not electrically shielded and are susceptible to electromagnetic interference, which is a problem in the environment of use in industries such as the aerospace industry. Furthermore, coatings and laminations which are typically used in the manufacture of piezo film contact transducers, such as Kynar.RTM., impede ultrasonic performance, thus making them generally unsuitable for ultrasonic contact transducer applications.
It is therefore desirable to provide an ultrasonic piezo film contact transducer that is flexible, is acoustically well matched to composite materials and is not susceptible to electromagnetic interference, but is inexpensive, lightweight, portable and easy to manufacture. It is also desirable to provide a structure for an ultrasonic film contact transducer that can easily and economically be employed to manufacture one piece arrays of transducers that have good acoustic and electric properties. The present invention achieves these goals.